Build an Amphibious Vehicle
An amphibious vehicle (or simply amphibian), is a vehicle or craft, that is a means of transport, viable on land as well as on water - just like an amphibian. This definition applies equally to any land and water transport, small or large, powered or unpowered, ranging from amphibious bicycles, ATVs, cars, buses, trucks, RVs, and military vehicles, all the way to the very largest hovercrafts. Classic landing craft are generally not considered amphibious vehicles, although they are part of amphibious assault. Nor are Ground effect vehicles, such as Ekranoplans. The former don't offer any real land transportation at all - the latter (aside from completely disconnecting from the surface, like an airplane), will likely crash on any but the flattest of landmasses. 
General technical notes
Apart from the distinction in sizes mentioned above, two main categories of amphibious vehicle are immediately apparent: those that travel on an air-cushion (Hovercraft) and those that don't. Amongst the latter, many designs were prompted by the desire to expand the off-road capabilities of land-vehicles to an "all-terrain" ability, in some cases not only focused on creating a transport that will work on land and water, but also on intermediates like ice, snow, mud, marsh, swamp etc.. This explains why many designs use tracks in addition to or instead of wheels, and in some cases even resort to articulated body configurations. Most land vehicles - even lightly armored ones - can be made amphibious simply by providing them with a waterproof hull and perhaps a propellor. This is possible thanks to the vehicle's volume usually being bigger than its displacement, meaning it will float. Heavily armored vehicles however sometimes have a density greater than water (their weight in kilograms exceeds their volume in liters), and will need additional buoyancy
measures. These can take the form of inflatable floatation devices,
much like the sides of a rubber dinghy, or a waterproof fabric skirt
raised from the top perimeter of the vehicle. For propulsion in or on the water some vehicles simply make do by spinning their wheels or tracks, while others can power their way forward more effectively using (additional) screw propellor(s) or water jet(s). Most amphibians will work only as a displacement hull when in the water - only a small number of designs have the capability to raise out of the water when speed is gained, to achieve high velocity hydroplaning, skimming over the water surface like speedboats. HistorySome of the earliest known amphibious vehicles were amphibious carriages, the invention of which is credited to the notorious Napolitan Prince Raimondo de Sangro of SanSevero (ca. 1750) or Sir Samuel Bentham (1781). The first known self-propelled amphibious vehicle, a steam-powered wheeled dredging barge, named the Orukter Amphibolos, was conceived and built by United States inventor Oliver Evans in 1805, although it is disputed to have successfully travelled over land or water under its own steam[1]. Although it is unclear who (and where and when) built the first combustion-engined amphibian, in all likelihood the development of powered amphibious vehicles didn't start until 1899. Until the late 1920s the efforts to unify a boat and an automobile mostly came down to simply putting wheels and axles on a boat hull, or getting a rolling chassis to float by blending a boat-like hull with the car's frame (Pohl, 1998). One of the first reasonably well documented cases was the 1905 amphibious petrol-powered carriage of T. Richmond (Jessup, Iowa, USA). Just like the worlds first petrol-powered automobile (1885, Carl Benz) it was a three-wheeler. The single front wheel provided direction, both on land and in the water. A three-cylinder petrol combustion-engine powered the oversized rear wheels. In order to get the wheels to provide propulsion in the water, fins or buckets would be attached to the rear wheel spokes. Remarkably the boat-like hull was one of the first integral bodies ever used on a car (Pohl, 1998). 
Since the 1920s development of amphibious vehicles greatly diversified. Numerous designs have been created for a broad range of applications, including recreation, expeditions, search & rescue, and military, leading to a myriad of concepts and variants. In some of them the amphibious capabilities are central to their purpose, whereas in others they are only an expansion to what has remained primarily a watercraft or a land vehicle. Small wheeled amphibiansAmongst the smallest non air-cushioned amphibious vehicles are amphibious bicycles, (see also) and ATVs. Although the former are still an absolute rarity, the latter saw significant popularity in North America during the nineteen sixties and early seventies. Typically an Amphibious ATV or AATV is a small, lightweight, off-highway vehicle, constructed from an integral hard plastic or fibreglass bodytub, fitted with six (sometimes eight) driven wheels, with low pressure, balloon tires. With no suspension (other than what the tires offer) and no steering wheels, directional control is accomplished through skid-steering - just as on a tracked vehicle - either by braking the wheels on the side where you want to turn, or by applying more throttle to the wheels on the opposite side. Most contemporary designs use garden tractor type engines, that will provide roughly 25 mph top speed on land. Constructed this way, an AATV will float with ample freeboard and is
capable of traversing swamps, ponds and streams as well as dry land. On
land these units have high grip and great off-road ability, that can be
further enhanced with an optional set of tracks that can be mounted
directly onto the wheels. Although the spinning action of the tires is
enough to propel the vehicle through the water - albeit slowly -
outboard motors can be added for extended water use. Current AATV
manufacturers are Argo, Land Tamer, MAX ATVs and Triton. Recently some efforts are made towards amphibious ATV's of the straddled variety. For instance in the form of an add-on inflatable pontoon kit, that can be installed on any quad-bike ATV with front and rear metal frame racks and at least 14" water fording ability. A new development was shown in 2006 by Gibbs Technologies. Their Quadski is a prototype for a cross between a Jetski and a Quad-bike. Amphibious cars
Amphibious automobiles have been conceived from ca. 1900, however the Second World War
significantly stimulated their development. Two of the most significant
amphibious cars to date were developed during World War II. The most
proliferous was the German Schwimmwagen, a small jeep-like 4x4 vehicle designed by the Porsche engineering firm in 1942 and widely used in World War II. The amphibious bodywork was designed by Erwin Komenda, the firm's body construction designer, using the engine and drive train of the Kübelwagen. An amphibious version of the Willys MB jeep, the Ford GPA or 'Seep' (short for Sea jeep) was developed during World War II as well. A specially modified GPA, called Half-Safe, was driven and sailed around the world by Australian Ben Carlin in the 1950s. Purely recreational amphibian cars include the 1960s Amphicar and the contemporary Gibbs Aquada. With almost 4.000 pieces built, the Amphicar is still the most successfully produced civilian amphibious car to date. The Gibbs Aquada stands out due to its capability of high speed planing on water. Other amphibious cars currently in production include the Dutton Commander 'AmphiJeep' (GB), the US Hydra Spyder and WaterCar, as well as several Chinese designs like the JMC BY5020TSL (see also) and BJ5032(XZHE), and the even longer JMC JX5021TLYDS. American distributor Rodedawg is now seeking to bring an adapted version of the Chinese amphibs to America. 
Amphibious busesAmphibious buses are employed in some locations as a tourist attraction.
Amphibious trucks and barges
With more than 20.000 units produced, the DUKW was the most successful amphibious truck of World War II. This 31-foot 6x6 truck was deployed in the Pacific theatre to establish and supply beachheads. It was designed as a wartime project by Sparkman & Stephens, the famous yacht design firm who also designed the hull for the Ford GPA 'Seep'. During the Vietnam War, the US Army used the amphibious articulated Gama Goat and the larger Caterpillar 'Goer' truck-series to move supplies through the canals and rice paddies of Southeast Asia. The latter was based on a 1950s civil construction vehicle and became the US Army’s standard heavy tactical truck before its replacement by the HEMTT. Although the vehicles' wheels were mounted without suspension or steering action, and land speeds over 20 mph were ill-advised, its articulated design provided it with good maneuverability and helped it to keep all four wheels firmly in touch with uneven ground. Coupled with its amphibious capability, in the Vietnam War (especially during raining season), the M520 Goer developed a reputation of being able to go where other trucks could not. For taking vehicles and supplies onto the beaches the US used the 1950s designed LARC-V and the huge LARC-LX or "BARC". At 63 feet long and 27 feet wide the latter is one of the largest wheeled amphibians to date. It could carry up to 100 tons of cargo or 200 people, but a more typical load was 60 tons of cargo or 120 people. The vehicle was powered by four V8 diesel engines positioned in the sides of the hull, each driving a single 8-foot wheel. The United Kingdom used the 6x6 wheeled Alvis Stalwart as their amphibious cargo carrier. This highly mobile 5-ton truck entered service with the British Army in 1966. In the water it was driven by vectored thrust water-jet propulsion units at about 6 knots. American manufacturer Terrawind currently offers civilian amphibious buses and motorcoaches. EWK Eisenwerke Kaiserslautern GmbH developed unique amphibious vehicle M3, that can be used as a ferry and as a floating bridge for trucks and heavy combat vehicles.
Wheeled armored vehicles
Many modern military vehicles, ranging from light wheeled command and reconnaissance, through armoured personnel carriers and tanks, are manufactured with amphibious capabilities. Contemporary examples of wheeled armored amphibians are the French Panhard VBL and GIAT Industries VAB.
Tracked unarmored vehiclesThe M29 Weasel (Studebaker Weasel), whilst originally designed as a snow vehicle, operated successfully in amphibious role by the addition of front and rear floats. The basic vehicle will float but its bow is square so the additional floats add stability and load carrying capacity. Tracked armored vehicles and tanks
Among tracked armored vehicles with amphibious capabilities are first of all those that are intended for use in amphibious assault. The United States started developing a long line of LVT (Landing Vehicle Tracked) designs from ca. 1940. The US Marine Corps currently uses the AAV7-A1 Amphibious Assault Vehicle, which is to be succeeded by the Expeditionary Fighting Vehicle (previously AAAV), which is capable of planing on water and can achieve water speeds of 37 - 46 km/h. Some heavy tanks have an amphibious mode in which a fabric skirt is needed to add buoyancy. The Sherman DD tank used in the D-Day invasion had this setup. When in water the waterproof float screen was raised and propellers deployed. The M2 and M3 Bradleys also need such a skirt. Some military vehicles are not truly amphibious but are capable of "wading" using waterproof screens to keep the upper hull dry. In World War II the tanks following the Sherman DDs were given waterproofed hulls and trunking was fixed to the engine intakes and exhausts to allow them to come ashore from landing craft in shallow water. The Germans gave their Tiger tank a long snorkel, essentially a long tube on the commanders hatch that allowed it to wade through four metres of water.
Articulated and multi-unit tracked amphibians
The unique capability that distinguishes multi-unit vehicles from single unit ones, is the ability to help each other. According to a 1999 article
in Military Parade magazine, multi-unit, all-terrain transport vehicles
were first proposed by the British in 1913, and by the 1950s, over 40
types of articulated tracked vehicles (ATVs) were in production.
The articulated tracked concept is chosen primarily for its combination
of high maneuverability, cross-country abilities, and remarkable
load-carrying capacity. In some cases the design is made amphibious,
giving them all-terrain capability in the truest sense. Usually the
front unit houses at least the engine, gearboxes, fuel tank(s) and the
driver's compartment, and perhaps there is some space left for cargo or
passengers, whereas the rear unit is the primary load carrier. A highly specialised development is the Arktos expedition and evacuation craft, that uses a linkage with two joints to connect the two units, as well as fitting each unit with its own engine, to give each unit enhanced independance of movement. Hovercraft
For certain applications wheeled and tracked amphibious vehicles are slowly being supplanted by Air-cushioned landing craft in many modern militaries. An Air-Cushion vehicle (ACV) or hovercraft is designed for traveling over land or water supported by a cushion of slow moving, low-pressure air ejected downwards against the surface below it. In principle a hovercraft can travel over any sufficiently smooth surface, solid, liquid, mixed, or anything in between. Considering that hovercraft can be quite large, some riding on an air-cushion contained by skirts several meters tall, these can deal with a reasonable level of unevenness in the terrain, unfazed by obstacles 1 to 2 meters in height. On the other hand the smallest personal hovercraft - ACVs no bigger than a compact hatchback - are nimble enough to follow some rolling of the terrain just as easily. One of the benefits of this type of amphibious craft is the possibility of making them large - the British-built SR-N4 Mk-3 Channel-crossing ferries were 56,4m (185 ft) in length and 23,8m (78 ft) wide. Other benefits of ACVs include their very high water speed (an SR-N4 Mk-1 could do 83 knots - 95 mph or 154 km/h !) and the fact that they can make the transition from land to water (or vice versa) at speed - contrary to most wheeled or tracked amphibians. Drawbacks are high fuel consumption and noise levels. For military purposes, the hovercraft's ability to distribute its laden weight evenly across the surface below it[clarify] makes it perfectly suited to the role of amphibious landing craft. The US Navy LCAC can take troops and materials (if necessary an M1 Abrams tank) from ship to shore and can access more than 70% of the world's coastline, as opposed to conventional landing craft, that have only about 17% of that coastline available to them for landing. Further reading
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